The oil industry is increasingly demanding multi-phase flow-meters for well testing and monitoring. A multi-phase flow-meter for such applications is required to measure the flow rates of liquid (oil and water) and gas in a oil well tubulars, such as production pipelines, flowlines, risers etc. In order to determine flow rates of multi-phase flows, it is generally necessary to measure the velocities and the holdups of the liquid and gas phases. The flow velocity is the velocity at which the phase, i.e., liquid or gas, travels measured in distance over time. The holdup of a phase is the fraction of the cross-section of the total flow occupied by the phase in question. The holdup is usually expressed in per cent. With a known given cross-section of the pipe, measured holdup and velocity, the flow rate of a phase can be calculated as the product of those three parameters.
The present generation of commercially available multi-phase flow-meters measure these flow rates by using a combination of measurement techniques, such as Venturi, positive displacement, gamma-ray, X-ray, microwave and electrical impedance. These meters, however, are all in-line types which need to be placed inside the well tubular or, at least, need a window into the tubular to perform the measurements.
The invention also relates to a type of meter which is defined as xe2x80x9cclamp-onxe2x80x9d type of multi-phase flow-meter. Using this type of meter, there is no need to interfere with the integrity of the flow tubular onto which the meter is mounted.
Compared with the in-line meters, a clamp-on type multi-phase flow-meter offers strong operational and economic advantages:
The device is applicable to either periodic testing or permanent installation. It can be clamped on to the outside of a tubular with no disruption of production, small crew size, small equipment size, no production down-time for maintenance in the case of permanent installation. However, any sensor or combination of sensors in a clamp-on meter will have to measure the phase flow-rates through steel pipes, and this excludes many techniques used in in-line meters.
Techniques which are considered appropriate for clamp-on measurement include nuclear(gamma-ray, X-ray and neutron), acoustic and ultrasonic methods. Although at present, there are clamp-on type gamma-ray systems used for multi-phase flow metering, they form only part of a multi-phase flowmeter and need to be combined with other in-line type of measurements to produce the three flow-rates. Clamp-on passive acoustic sensors, in combination with other techniques, have been used to meter multi-phase flows.
In U.S. Pat. No. 5,415,048 a clamp-on acoustic system is described which, when used in combination with a differential pressure measurement system, produces the mass flow-rates of the liquid and gas phases. The complete system, however, does need pressure tapings on the pipeline, in order to take the differential pressure measurement. The other disadvantage of the passive acoustic measuring method is the poor signal to noise ratio at relatively low mass flow rates.
Ultrasonic clamp-on flowmeters are commercially available. Most of these are based on the transit time method, measuring upstream and downstream travel times of ultrasound pulses. An example are the meters made by Endress +Hauser Flowtec AG and described in U.S. Pat. No. 5,533,408. The known meter combines a transit time measurement system with a cross-correlation system. The former is for clean fluids whereas the latter is for fluids containing foreign particles (reflectors). An automatic switch selects one or the other system depending upon the concentration of the reflectors.
The U.S. Pat. No. 4,735,097 shows a ultrasonic flow measurement system based employing a plurality of transducers located at two cross-sectional portions of a pipeline.
The U.S. Pat. No. 4,735,097 (or EP-A-0212470) and U.S. Pat. No. 4,838,127 describe the generation of surface waves in the pipe wall for clamp-on type ultrasonic flowmeters. It appears that the known flowmeters are using surface waves to increase the effective area of the source (and receiver). However, the flow itself is characterized by measuring the attenuation or travel time of longitudinal waves travelling through the liquid.
It is therefore an object of the invention to provide a flowmeter particularly suitable for monitoring streams of formation fluid as produced from subterranean hydrocarbon reservoirs.
The flowmeter of the present invention is based on measurements of the attenuation of various acoustic wave modes propagating in the pipe wall, in response to the presence of a fluid phase or mixtures of fluid phases in the pipe. Preferably, the flowmeter is designed for flows subject to gravity separation, i.e., non-vertical flows. It assumes an asymmetrical distribution of the phases in the flow.
Various wave modes can be excited in the pipe wall by appropriate setups of ultrasonic transducers mounted on the pipe periphery. These modes include those traveling along the axial direction, the circumferential direction and the radial or thickness direction of the pipe wall. The wave modes can be so-called bulk waves, i.e., compressional or shear wave modes, or various guided waves, in particular Lamb wave modes.
The bulk wave modes of interest to the present invention are those that are reflected between inside and outside surface of the pipe wall and hence travel in axial or circumferential direction along a zigzag path. The main radial mode is a compressional wave bouncing between the two surfaces of the pipe wall in the thickness direction, referred to as thickness mode reverberation in the wall. Lamb waves on the other hand are guided waves within the boundary of the two surfaces of the pipe wall. They are also referred to as xe2x80x9cplatexe2x80x9d waves and are sometimes classified as a subgroup of the so-called surface or Raleigh waves.
Many of the modes mentioned above, whether axial, circumferential or radial, are of leaky nature, i.e. they leak energy into the fluid phases, predominantly into any liquid phase present in the flow, that is in contact with the pipe wall. The amount of energy loss depends on the acoustic impedance of the liquid phase as well as on the fraction it occupies in the pipe, i.e. the liquid holdup. Therefore, by measuring the decay rates of appropriate wave modes in the pipe wall, the acoustic impedance and the holdup of the fluid phase/s can be determined. The appropriate wave modes are combined for the purpose of the present invention under the term xe2x80x9cin-wall leaky acoustic wave modesxe2x80x9d.
In a multi-phase flow, the liquid holdup fluctuation causes fluctuations in the attenuation rate of the in-wall wave modes. These fluctuations provide tag signals (a characteristic temporal signature) of the flow. Therefore a time of flight measurement can be performed with two of such attenuation measuring channels arranged along the flow direction with known separation between them. A flow velocity can thus be obtained by combining the measured time delay and the separation.
Based on the above basic principle, the flowmeter comprises at least one but preferably two or more ultrasonic transmitter-receiver groups arranged on the outer surface of the pipe and with known separations between them. Each group comprises at least of one transmitter, which emits ultrasonic energy and generates axially, circumferentially propagating and/or radially reverberation wave modes in the pipe wall, and of at least one receiver, which detects the attenuation of the ultrasonic energy after propagating along the pipe wall for a given distance. The receiver can be implemented either by the same transducer that also functions as the transmitter or by one or more separate transducers mounted at different locations around the pipe periphery. Different combinations of the detected signals from one or multiple receivers can be utilized to obtain information on the holdup of the fluid phase/s and on distribution of the acoustic impedance in the pipe. As discussed below, there are various ways of measuring the acoustic impedance distribution and the liquid holdup.
The measurement of the fluid holdup can be combined with velocity measurements as obtained by the cross-correlation method to determine flow rates of individual phases. The accuracy of the measurements is determined by the wave mode, frequency, path geometry, combined with sufficient sensitivity to fluid impedance. Using the new method, information on fluid impedance can be readily generated through processing of the received signal.
Using axial Lamb waves with multiple transmitters and receivers, the fluid impedance distribution around the circumference of the pipe can be scanned. In a horizontal multi-phase flow with more or less stratified phase separation, this leads to the generation of a holdup distribution profile of the flow. This array of transmitter-receiver pairs serves as a multi-phase fraction meter.
When combined with the impedance cross-correlation and Doppler methods, a gas/oil/water three-phase flowmeter is produced. Alternatively, holdup profiles obtained from more than one of such arrays along the flow direction can be cross-correlated to produce a velocity profile. The combination of the holdup and velocity profiles produces the flow rates of individual phases.
The flowmeter in accordance with the invention operates as a non-intrusive device. It can be mounted as a clamp-on design or attached permanently to a flow tubular or pipe. In the field of production of hydrocarbons, its possible applications range from use as a surface testing device, over logging applications to permanent downhole flow monitoring by installation on a production tubing or by placing it into the space behind the casing.
Compared with the prior work on clamp-on cross-correlation flowmeters, the advantage of this invention is that the ultrasonic energy propagation path is in the pipe wall, rather than through the fluid. Therefore this method will work even at very high gas flow rates, where the other methods fail due to increased gas bubble concentration in the liquid phase, leading to the complete blockage of the through flow transmission. The applications are mainly aimed at horizontal pipes where the flow regimes are mainly slug/bubbly or wavy stratified. This invention, combined with an appropriate flow model, enables measurements of the flow rates of the individual phases in a multi-phase flow.
These and other features of the invention, preferred embodiments and variants thereof, and further advantages of the invention will become appreciated and understood by those skilled in the art from the detailed description and drawings following below.